The present invention relates to a semiconductor device including a plurality of semiconductor elements stacked to each other and a method of fabricating the semiconductor device, and particularly to a technique capable of stacking semiconductor elements to each other without any limitation by outer sizes of the semiconductor elements.
If one semiconductor element is mounted on a wiring board, then an area of the wiring board is occupied with the semiconductor device, and thereby another semiconductor element is no longer mounted on the wiring board. On the other hand, in recent years, electronic devices such as video cameras, CDs, and cellular phones have been required to be further reduced in size and further enhanced in performance. To meet such a requirement, there has been proposed a semiconductor device, in which a semiconductor element mounting area becomes twice that of a prior art semiconductor device although the semiconductor device uses the same wiring board as that used for the related art semiconductor device.
For example, a related art semiconductor device 1 shown in FIG. 4 includes two semiconductor elements 3 and 5, wherein a surface (back surface) 9 opposed to an electric connection surface 7 of the semiconductor element 3 is superimposed to a surface (back surface) 13 opposed to an electric connection surface 11 of the other semiconductor element 5 and is bonded thereto by means of adhesive 15, and the electric connection surface 11 of the upper semiconductor element 5 is electrically connected to segments of wiring on a wiring board 19 by means of bonding wires 17 while the electric connection surface 7 of the lower semiconductor element 3 is electrically connected to segments of the wiring on the wiring board 19 by means of bumps 23.
Another related art semiconductor device 25 shown in FIG. 5 includes two semiconductor elements 27 and 29, in which a surface (back surface) 33 opposed to an electric connection surface 31 of the semiconductor element 27 is die-bonded to a wiring board 19 by means of adhesive 15 and electrodes of the semiconductor element 27 are electrically connected to segments of wiring on the wiring board 19 by means of bonding wires 17, and the other semiconductor element 29 is bonded, in a flip-chip bonding manner, to a front surface of the semiconductor element 27 by means of bumps 35.
With each of the semiconductor devices 1 and 25, a mounting density of the semiconductor device becomes twice that of a conventional semiconductor device in which a mounting area is occupied with one semiconductor element. Accordingly, it is possible to miniaturize an electronic device using the semiconductor device.
The related art semiconductor device 1 shown in FIG. 4, however, has an inconvenience that an outer size of the lower layer semiconductor element must be larger than an electrode area, in which electrodes are disposed, of the upper layer semiconductor element. The reason for this is as follows: namely, at the time of connecting the bonding wires to respective electrodes of the upper layer semiconductor element, some support is required to be disposed directly under each of the electrodes of the upper layer semiconductor element. If such a support is not provided (that is, in an overhang state), when the bonding wire is connected to each electrode of the upper layer semiconductor element, a mechanical load is partially applied to the upper layer semiconductor element, resulting in breakage of the upper layer semiconductor element.
The related art semiconductor device 25 shown in FIG. 5 has also an inconvenience that since the bonding wires must be connected to the lower layer semiconductor element, an outer size of the upper layer semiconductor element is required to be smaller than an electrode area, in which the electrodes are disposed, of the lower layer semiconductor element.
Accordingly, in each of the above-described related art semiconductor devices, there is a limitation to a relationship between the outer sizes of the lower layer semiconductor element and the upper layer semiconductor element. As a result, depending on a combination of outer sizes of semiconductor elements, the semiconductor devices cannot be stacked to each other, thereby failing to realize high density mounting of the semiconductor elements.
An object of the present invention is to provide a semiconductor device capable of mounting semiconductor elements of arbitrary outer sizes to each other without any limitation by the outer sizes of the semiconductor elements, thereby realizing, even in a combination of any outer shapes of semiconductor elements vertically stacked to each other, high density mounting of the semiconductor elements, and to provide a method of fabricating the semiconductor device.
To achieve the above object, according to a first aspect of the present invention, there is provided a semiconductor device including: a first semiconductor element to be bonded to a wiring board in a flip-chip bonding manner; a resin peripheral wall formed on the wiring board in such a manner as to surround the first semiconductor element; a sealing resin poured so as to fill a space surrounded by the resin peripheral wall and then hardened; and a second semiconductor element provided in such a manner that a back surface thereof is fixed on an upper surface of the sealing resin and an electrode provided on a front surface thereof is connected to a segment of wiring on the wiring board by means of a bonding wire.
With this configuration, even if an outer size of the second semiconductor element is larger than that of the first semiconductor element, the second semiconductor element can be placed on the sealing resin. Accordingly, in the bonding step, a mechanical load applied to the second semiconductor element is supported by the sealing resin, so that it is possible to prevent occurrence of breakage of the second semiconductor element. As a result, the second semiconductor element of an arbitrary outer size can be stacked on the first semiconductor element without any limitation by the outer size of the first semiconductor element. That is to say, even in a combination of any outer shapes of semiconductor elements vertically stacked to each other, the semiconductor elements can be mounted at a high density.
In this semiconductor device, preferably, a portion, in the thickness direction, of the second semiconductor element is buried in the sealing resin, and the back surface of the second semiconductor element is supported on the back surface of the first semiconductor element via the sealing resin, and the front surface of the second semiconductor element is projected from the upper surface of the sealing resin.
With this configuration, since a portion, in the thickness direction, of the second semiconductor element is buried in the sealing resin, an adhesive strength of the second semiconductor element to the sealing resin can be improved. Also since the back surface of the second semiconductor element is supported on the back surface of the first semiconductor element via the sealing resin, it is possible to prevent the second semiconductor element from being excessively buried in the sealing resin and hence to realize accurate positioning of the second semiconductor element in the height direction. Further since the front surface of the second semiconductor element projects from the front surface of the sealing resin, it is possible to prevent electrodes provided on the front surface of the second semiconductor element from being covered with the sealing resin.
According to a second aspect of the present invention, there is provided a method of fabricating a semiconductor device, including the steps of: bonding a first semiconductor element on a wiring board in a flip-chip bonding manner; forming a resin peripheral wall on the wiring board in such a manner that the first semiconductor element is surrounded by the resin peripheral wall; filling a space surrounded by the resin peripheral wall with a liquid sealing resin; fixing a back surface of a second semiconductor element on an upper surface of the sealing resin; and connecting a electrode provided on a front surface of the second semiconductor element to a segment of wiring on the wiring board by means of a bonding wire after fixture of the second semiconductor element on the sealing resin.
With this configuration, after the resin peripheral wall is formed on the wiring board in such a manner as to surround the first semiconductor element, the space surrounded by the resin peripheral wall is filled with the sealing resin. Accordingly, the sealing resin containing the first semiconductor element and having the outer shape defined by the resin peripheral wall is formed into a bed plate shape on the wiring board, wherein an upper surface of the sealing resin is taken as a mounting surface for supporting the second semiconductor element, and the second semiconductor element is fixed on the upper surface of the sealing resin. As a result, the second semiconductor element can be supported by the sealing resin irrespective of an outer size of the first semiconductor element. That is to say, it is possible to fabricate the semiconductor device in which semiconductor elements of any shapes are allowed to be stacked to each other in the vertical direction.
The above step of fixing the second semiconductor element on the upper surface of the sealing resin is preferably carried out by placing the back surface of the second semiconductor element on the upper surface of the sealing resin before perfect hardening of the sealing resin and after the sealing resin becomes hard by a prescribed level.
With this configuration, before perfect hardening of the sealing resin and after the sealing resin becomes hard by a prescribed level, the second semiconductor element is placed on the upper surface of the sealing resin and is fixed thereto. In other words, at the same time when the sealing resin is perfectly hardened, the second semiconductor element is fixed to the sealing resin. Accordingly, it is possible to eliminate the need of special-purpose adhesive for fixing in the case of fixing the second semiconductor element to the sealing resin after perfect hardening.
A viscosity of a resin filled as the sealing resin is preferably lower than a viscosity of a resin used for forming the resin peripheral wall.
With this configuration, since the viscosity of the sealing resin is lower than the viscosity of the resin used for forming the resin peripheral wall, it is possible to increase a filing ratio of the sealing resin in the space of the resin peripheral wall. Conversely, since the viscosity before hardening of the resin used for forming the resin peripheral wall is higher than the viscosity before hardening of the sealing resin, it is possible to prevent flow-out of the sealing resin from the resin peripheral wall.